HK1131179A1

HK1131179A1 - Cholesteric multi-layers

Info

Publication number: HK1131179A1
Application number: HK09109032.2A
Authority: HK
Inventors: Michael Kasch; Adolf Gurtner
Original assignee: Sicpa Holding Sa
Priority date: 2006-06-27
Filing date: 2007-06-27
Publication date: 2010-01-15
Also published as: CA2653576A1; EA200900062A1; EP2032672B1; EA017862B1; JP5261799B2; BRPI0713468A2; AU2007263738B2; KR101483800B1; EP1876216A1; EP2032672A1; PL2032672T3; ES2349344T3; US8142682B2; VN22663A1; CN101479362B; TW200808942A; WO2008000755A1; TWI409319B; US20100178508A1; CA2653576C

Abstract

The invention discloses a multilayer of cholesteric liquid crystal polymer (CLCP), wherein at least two layers of CLCP, differing in at least one optical property, are arranged on top of each other, characterized in that said at least two layers are chemically cross-linked together through the polymer network, such as to mechanically form a unique solid body. Corresponding pigments, coating compositions and there use in security and decorative printing and coating applications are disclosed as well.

Description

Cholesteric multilayer

Technical Field

The invention belongs to the field of special pigments for coating compositions, in particular for printing inks for security documents. The present invention proposes a new cholesteric liquid crystal polymer layer and the resulting pigment that allow for a higher degree of change in spectral reflectance characteristics, significant reflected color, and angle-dependent color change.

Background

Films and pigments produced from Cholesteric Liquid Crystal Polymers (CLCP) are known in the art. See US 5,211,877(Andrejewski et al); US 5,362,315(Muller-Rees et al); and US 6,423,246(Kasch et al) which discloses compositions and techniques for producing such materials.

Cholesteric liquid crystal polymers exhibit a molecular order in the form of a stack of spirally arranged molecules. This order state is the origin of a periodic refractive index modulation throughout the liquid crystal material, which in turn leads to wavelength-determined light selective transmission/reflection (interference filter effect). The particular position of the helical molecular arrangement in the CLCP causes the reflected light to be circularly polarized, left-handed or right-handed, depending on the direction in which the molecular helix is rotated.

As known to those skilled in the art, the wavelength range reflected by a CLCP is determined by its geometry of periodic refractive index modulation (i.e., the pitch of the molecular helix). For a given cholesteric liquid crystal precursor material, the pitch depends on a series of optional factors, among which temperature, as well as the quantitative presence of solvent and defined chirality-inducing additives; thus, the wavelength of maximum reflection can be determined by the chosen manufacturing method. The pitch of the material can be finally frozen by a cross-linking (polymerization) reaction, so that the color of the resulting Cholesteric Liquid Crystal Polymer (CLCP) is no longer dependent on external factors.

To this end, the monomeric or oligomeric cholesteric liquid-crystal material is provided with reactive groups, for example acrylate and/or methacrylate residues, which can undergo a crosslinking reaction in the presence of suitable photoinitiators under the influence of UV radiation. Thus, freezing the pitch of a suitably oriented CLCP precursor can be performed simply by exposure to ultraviolet light (UV-curing).

In addition to the defined reflection color, the Cholesteric Liquid Crystal Polymer (CLCP) also shows a more or less pronounced viewing angle-dependent color change ("color shift"). Films and pigments made from CLCP are therefore used as security elements on value and identity documents, since the colour-shift effect cannot be reproduced by copiers. The reflection band of a CLCP material is relatively narrow and its angular dependence is given by

λ_refl.＝n*p*cos(α)

Wherein λ_refl.Is the maximum reflection wavelength; n is the average refractive index of the material (about 1.5); p is the pitch of the molecular helix; and α is the viewing angle (Eberle et al, Liq. Crystal.1989, Vol.5, No 3, 907-. It is deduced from this equation that an increase in viewing angle causes the reflected wavelength to shift to shorter wavelengths.

Many different reflection colors can be achieved with the same given CLCP precursor material by appropriate selection of fabrication conditions. Still further, the chirality (left-or right-handed) of the reflection may also be selected by appropriate selection of chirality-inducing additives in the fabrication of the material. However, in the field of pigments for security printing, an increase in the number of physically realizable properties is seen as an advantage in terms of serving many different security document applications.

The number of different optical reactions, i.e. "color" and "color shift", that can be achieved can be greatly increased if different CLCP pigment types with different optical reactions are combined with each other in the same ink. The generation of the security element in this case depends on the availability of two or more different pigments which are mixed together in suitable proportions for serving a defined security document application.

It can be seen that if different optical reactions can be combined into the same physical pigment, it is possible to further improve the security level of the CLCP material, since it is much easier to manufacture an ink comprising a mixture of several modulated pigments with basic optical reactions (i.e. to combine letters of the alphabet) than to manufacture a single pigment combining optically basic reactions into a more complex reaction (i.e. to find a definite word). While the former can be done in essentially any printer shop, if basic paint is available, the latter can only be done on the paint manufacturing equipment, and thus enables perfect control of the paint supply chain.

Cholesteric polymer multilayers consisting of laminated monolayers have previously been described by Dobrusskin et al in WO 95/08786. This document discloses a coloured material comprising a first type of aligned Chiral Liquid Crystal Polymer (CLCP) layer and a second type of aligned Chiral Liquid Crystal Polymer (CLCP) layer, each layer reflecting light in a respective wavelength region when viewed at a given angle and being solid at room temperature.

To prepare the coloured material of WO 95/08786, the CLCP precursor of the first layer L1 is mixed with a photoinitiator and spread on the flexible carrier sheet S at a first temperature T1 such that the CLCP precursor aligns to form a first colour. The CLCP precursor is then crosslinked by exposing the layer to ultraviolet radiation at the first temperature T1. A second layer L2 was prepared in the same manner and spread over the first layer L1 at a second temperature T2 such that the CLCP precursor aligned to form a second color and the CLCP precursor crosslinked by exposing the layer to ultraviolet radiation at the second temperature T2. An embodiment is disclosed having a first layer that shifts from infrared to red and a second layer that shifts from blue to ultraviolet, thus creating a device whose color shifts from blue to red when viewed from normal to oblique.

However, the two-layer material of WO 95/08786 has a significant disadvantage in that it cannot be ground to pigment. The manufacture of CLCP pigments involves peeling off a polymerized cholesteric layer from the carrier sheet using methods known to those skilled in the art, and then milling it to a pigment size suitable for use in ink and coating compositions. The bilayer material of WO 95/08786 is not amenable to the milling process described and therefore breaks down (delaminates) into its individual layers when the material is removed from the support sheet, or at least under the influence of the high energy input in a jet mill, rather than appearing as a single solid layer throughout the process. Thus, using the method and materials disclosed in WO 95/08786, it is not possible to prepare pigments having specific optical properties from cholesteric multilayers.

In US 2005/266158, liquid crystals are described, such as optical films or reflective polarizers. No pigments are contemplated in the reference. The optical film contains up to three different optical layers physically produced from a single coating on a substrate by subjecting the coating to a series of solvent evaporation and UV curing steps. However, the process of US 2005/266158 is not well suited for industrial production due to the need to evaporate the solvent, for health, safety and environmental concerns.

The object of the present invention is to overcome the disadvantages of the prior art and to provide pigments having special, hitherto unavailable optical properties.

Summary of The Invention

The above object has been solved according to the invention by a cholesteric liquid crystal polymer multilayer, wherein at least two cholesteric liquid crystal polymers differing in at least one optical property are arranged on top of each other, characterized in that said at least two layers are chemically inter-layer crosslinked by means of a polymer network to form a mechanically unitary solid body which can be comminuted into a pigment without deteriorating its internal structure and which has a steep pitch of the cholesteric liquid crystal at the interface between said at least two layers of cholesteric liquid crystal polymer.

According to the invention, it was found that such a multilayer stack can be comminuted to pigments without any deterioration of its internal structure, so that pigments having advantageous, hitherto unachievable optical properties can be prepared.

According to the present invention, there are thus provided novel cholesteric multilayer materials and pigments prepared therefrom, which are capable of exhibiting advantageous, hitherto unattainable optical properties, such as high brightness and viewing-angle-dependent color change (color-flip effect), and special reflective properties, such as a color change from a short-wavelength color to a long-wavelength color when changing from direct to oblique viewing, or an extremely long run in the color space when responding to a change in viewing angle. According to the invention, the optical properties can be adjusted very precisely.

According to the present invention it was found that the above-described CLCP multilayer pigments can be obtained by providing mechanical delamination resistance through a specific selection of process conditions during the manufacture of the multilayer material.

To avoid mechanical delamination of the composite pigment made from the individual layers, it has been found to be a mandatory requirement to provide a sufficient amount of chemical crosslinking (interlayer crosslinking) between the individual layers. The prior art materials, such as the material manufactured according to WO 95/08786, do not have sufficient interlayer crosslinking because the reactive functional groups in each individual layer of the material are fully polymerized before the next layer is deposited on that layer. Thus, in the material of WO 95/08786, interlayer adhesion is provided only by mechanical and van der waals forces, and not by chemical bonds.

The different optical property is preferably the wavelength of the maximum reflection and/or the circular polarization state. However, it may also comprise light-absorbing or light-emitting properties, for example, which can be obtained by mixing dyes, pigments or light-emitting compounds into one of the CLCP layers of the multilayer.

Furthermore, the multilayer may contain additives with non-optical properties, such as magnetic particles, radio frequency resonance particles or forensic markers.

According to a first embodiment of the invention, the interlayer crosslinking is achieved by staggered curing (polymerization) as listed below:

applying the first layer L1 onto a flexible carrier foil as known to the person skilled in the art, but only partially curing the applied film. Typically, this layer is sufficiently cured to freeze the pitch of the CLCP material while still retaining a portion of the reactive groups originally present, which is sufficient for crosslinking with the second layer L2 applied thereon. The partial curing can be achieved by metered low dose uv irradiation and/or preferably by using less than the required amount of photoinitiator in the precursor composition of the L1 layer.

In a second step, a second layer L2 was applied on the L1 layer and the entire assembly was now cured thoroughly. Thorough curing is achieved by thorough uv irradiation, preferably using more than the required amount of photoinitiator together in the precursor composition of the L2 layer.

An optional step for depositing an additional layer of the first, partially curable coating (L1a, L1b, L1c.) may be inserted before applying the L2 layer, as required.

The product resulting from this process appears mechanically as a single solid polymer layer (the only solid) that optically exhibits the combined reflective properties of all of the individual layers that make up it (as will be described in more detail below with reference to the preferred embodiment of fig. 6).

The resulting product is further characterized in that it has a cholesteric liquid crystal pitch dip at the interface between the individual layers having different optical properties. This abrupt change is a distinguishing feature of the products of the invention and can be seen in the development of the cholesteric pitch across the multilayer (as will be described in more detail below with reference to the preferred embodiment of fig. 3); it is noteworthy that the pitch responsible for the optical interference properties (reflection wavelength) of the cholesteric material changes abruptly at the layer interface of the product of the invention. For example, in the preferred embodiment of fig. 3, there is a first pitch of about 200 nanometers in the left portion of the layer and a second pitch of about 130 nanometers in the right portion of the layer. The change from the first pitch to the second pitch occurs in less than one pitch height, so that no intermediate pitch is observed.

Thus, according to the invention, the term "abrupt change in cholesteric liquid crystal pitch" is defined as a change in cholesteric liquid crystal pitch at an interface between optical layers of a body of the invention, from a first value of cholesteric liquid crystal pitch that is constant throughout the first optical layer at the interface to a second value of cholesteric liquid crystal pitch that is constant throughout the second optical layer at the interface, the change occurring in less than one pitch height such that no intermediate pitch is observed.

The constancy of the cholesterol pitch throughout the optical layer can be determined statistically, for example, from the lack of slope in a linear regression of the pitch height p versus the pitch number n, in terms of p ═ a × n + b. If the experimentally determined slope (a) is higher than three times its standard deviation sigma (a), 99.7% make sure that the slope is not zero, i.e. that the pitch is not constant. Otherwise, the pitch may be assumed to be constant.

This sudden, stepwise change in the liquid crystal pitch at the interface of the optical layers is the result of the particular manufacturing process that produces the product of the present invention and is different from the product of US2005/0266158a1, in that the latter contains up to three different optical layers physically produced from a single coating on a substrate by subjecting the coating to a series of solvent evaporation and UV curing steps. The method obviously does not produce sudden pitch changes. Instead, a more or less gradual pitch change through the liquid crystalline polymer layer is obtained, which can be easily seen by scanning electron micrographs.

As a result of the manufacture, the cholesteric structure of the product of the invention has a constant first pitch value (within the statistical fluctuation range) corresponding to the first reflection wavelength throughout the entire thickness of the first optical layer, subsequently a constant second pitch value (within the statistical fluctuation range) corresponding to the second reflection wavelength throughout the entire thickness of the second optical layer, etc. The product of the invention has a defined, stepwise level of cholesterol pitch values and there are no smooth changes as in the product of US 2005/0266158.

In the present invention, the first polymerization step is carried out so as to leave sufficient reactive groups that they can undergo a crosslinking reaction with adjacent layers during the subsequent polymerization step. The result is a fully crosslinked polymer film in which no phase boundaries are present.

In an alternative way of achieving the bilayer or multilayer structure of the invention, successive coatings of the respective cholesteric liquid crystal precursor composition are applied in a single step on a flexible carrier foil. The composition is applied to the support in the molten state by means of an in-line coating station (in-line coating station) and the respectively applied coating is immediately cooled down, with the aim of freezing the liquid-crystal mixture in situ and avoiding its mixing with the next coating applied thereon. The orientation and curing (polymerization) of the entire composite coating is carried out once at the final curing station (co-curing). The thickness of the various layers is as in the first embodiment and will be described in more detail below.

In a variant of the given embodiment of staggered and co-curing, the coating is carried out using a solution of the CLCP monomeric precursor material in an organic solvent or solvent mixture (wet coating), whereby the solvent is evaporated (dried) after each coating operation.

In another variation of the given embodiment, a continuous strip of heat resistant material (e.g., steel, aluminum, etc.) is used as the support for the coating. This enables the processing of CLCP precursors having a liquid crystalline phase at temperatures ranging up to 400 ℃.

The CLCP precursor deposited in any given embodiment may be protected by a cover foil of PET or any other suitable material, intended to exclude atmospheric oxygen during the curing step. The cover foil must be sufficiently thin and made of a suitable material so as not to absorb the UV radiation used for curing.

The curing of the polymer may be carried out under inert conditions (i.e. under an inert gas such as nitrogen, carbon dioxide or argon); this is required in particular in the case of electron beam curing to prevent oxidation reactions. In the case of inert conditions, the cover foil is no longer required for excluding oxygen.

Thus, the inventive method for preparing the above CLCP multilayer comprises: the CLCP monomeric precursor materials comprising crosslinkable groups of at least two coating layers are sequentially deposited on a flexible carrier substrate in such a way that they are on top of each other, and then the entire assembly is completely cured, thereby crosslinking substantially all crosslinkable groups throughout the coating layers, thereby forming a mechanically unitary solid having a steeply changing cholesteric liquid crystal pitch at the interface between the at least two layers of cholesteric liquid crystal polymer. The alternative method is different in that: according to a first variant, each CLCP coating is oriented and partially cured after deposition, leaving an amount of crosslinkable groups in the layer sufficient to chemically crosslink with adjacent coating layers, thereby forming a mechanically unitary solid having a steep cholesteric liquid crystal pitch at the interface of the at least two cholesteric liquid crystal polymers. On the other hand, according to a second variant, after deposition, each CLCP coating is freeze-dried or evaporation-dried. According to a second variant, the orientation of the CLCP coating is carried out after the deposition of all the coatings, by tempering the whole assembly, before the step of curing the whole assembly completely.

In addition to different colors and color shifts, a variety of other optical properties can be produced in the CLCP materials of the present invention, which are invisible to the unaided human eye and only noticeable with the aid of suitable instrumentation.

Narrow band spectral reflectance is an inherent property of CLCP materials having a highly regular pitch, and much effort has been expended in the prior art to increase the spectral reflectance bandwidth of CLCP pigments with the aim of obtaining brighter reflected colors, and therefore more attractive pigments. The bandwidth of the spectral reflection of the CLCP material can be increased by introducing random or progressive pitch changes during the manufacturing process via appropriate manipulation. This has for the first time become possible under the teaching of the present invention.

The method and material of the present invention allow a more accurate generation of a determined CLCP spectral reflection spectrum, since said spectrum can now be composed accurately by overlapping a suitable number of layers having respective characteristic narrow-band reflection spectra at preset wavelengths. This obviously allows to encode pigments with invisible, narrow-band spectral characteristics that do not show up as a visible appearance, but can be visualized by means of a spectrometer or special optical filter device.

The fact that the reflected light of the CLCP has circular polarization can be utilized as a further security element. The direction of such circular polarization is obviously determined by the manufacturing method. The handedness of circular polarization can be selected individually for each layer of the CLCP multilayer of the invention and this handedness of polarization can be manifested by means of a corresponding polarizing filter device. It is thus possible to impart to any of the layers of the multilayer CLCP a respective narrow-band reflection color and a respective polarization chirality.

The multilayer foil of the present invention is useful for many types of security and decorative applications. Preferably, the multilayer foil is used as a laminate for a security thread, either with a hologram orSimilar in form of foil security elements for protecting banknotes, certificates or other securities or identity documents.

Most preferably, the multilayer foils of the invention are made into pigments for use in ink and coating compositions for various security and decorative coating applications, such as security inks for value documents and identity documents, inks for artistic and commercial printing applications, paints for decorative coatings, and various cosmetic products (nail varnishes, pressed powders, etc.). In addition to this, the pigments can be incorporated into various types of plastic articles.

Detailed Description

The multilayer stack of the invention is made of a CLCP composition generally known to the skilled person.

Preferred CLCP compositions of the present invention comprise (weight percent (wt%) refers to total solids content):

A)20-99.5 wt.%, preferably 60-99 wt.%, of at least one or several three-dimensional crosslinkable compounds of the average general formula (1)

Y¹-A¹-M¹-A²-Y² (1)

Wherein

Y¹、Y²Are identical or different and represent polymerizable groups, such as acrylate, methacrylate, epoxy, isocyanate, hydroxyl, vinyl ether or vinyl residues;

A¹、A²is of the formula C_nH_2nWherein n is an integer from 0 to 20, and wherein one or several methylene groups may be replaced by oxygen atoms;

M¹having the formula-R¹-X¹-R²-X²-R³-X³-R⁴-；

Wherein

R¹To R⁴Are identical or different divalent residues selected from the group consisting of-O-, -COO-, -COHN-, -CO-, -S-, -C.ident.C-, -CH ═ CH-, -N ═ N-, -N ═ N (O) -and a C-C bond; and wherein R²-X²-R³Or R²-X²Or R²-X²-R³-X³May also be a C-C bond;

X¹to X³Are identical or different residues and are selected from the group consisting of 1, 4-phenylene; 1, 4-cyclohexylene; having 6 to 10 atoms in the aryl nucleus and 1 to 3 heteroatoms from O, N and S and carrying substituents B¹、B²And/or B³The heteroarylene group of (a); having 3 to 10 carbon atoms and carrying a substituent B¹、B²And/or B³Cycloalkylene of (a);

wherein

B¹To B³Are identical or different substituents selected from the group consisting of hydrogen, C₁-C₂₀Alkyl radical, C₁-C₂₀-alkoxy, C₁-C₂₀Alkylthio radical, C₁-C₂₀-alkylcarbonyl group, C₁-C₂₀-alkoxycarbonyl, C₁-C₂₀Alkylthio-carbonyl (alkylthiocarbonyl), -OH, -F, -Cl, -Br, -I, -CN, -NO₂Formyl, acetyl and alkyl-, alkoxy-or alkylthio-residues having a chain of 1 to 20 carbon atoms interrupted by ether oxygen, thioether sulfur or an ester group;

B)0.5 to 80 wt.%, preferably 3 to 40 wt.%, of at least one chiral compound of the average formula (2)

V¹-A¹-W¹-Z-W²-A²-V² (2)

Wherein

V¹，V²Identical or different and represent the following residues: acrylate, methacrylate, epoxy, vinyl ether, vinyl, isocyanate, C₁-C₂₀Alkyl radical, C₁-C₂₀-alkoxy, C₁-C₂₀Alkylthio radical, C₁-C₂₀-alkylcarbonyl group, C₁-C₂₀-alkoxycarbonyl, C₁-C₂₀-alkylthio carbonyl, -OH, -F, -Cl, -Br, -I, -CN, -NO₂Formyl, acetyl and alkyl-, alkoxy-or alkylthio-residues having a chain of 1 to 20 carbon atoms interrupted by ether oxygen, thioether sulfur or an ester group, or a cholesterol residue;

A¹、A²as described above;

W¹、W²having the formula-R¹-X¹-R²-X²-R³-；

Wherein

R¹、R²、R³As described above, and wherein R²Or R²-X²Or X¹-R²-X²-R³May also be a C-C bond;

X¹、X²as described above;

z is a divalent chiral residue selected from dianhydroxites (e.g., isosorbide or isomannide), hexoses, pentoses, binaphthyl derivatives, biphenyl derivatives, derivatives of tartaric acid, and optically active diols, and wherein V is¹Or V²A C-C bond in the case of a cholesterol residue.

These compositions are already known and are described in the art together with the processes for their manufacture, for example in EP 1149823 or in EP 1046692.

Particularly preferred Liquid Crystal (LC) mixtures according to the invention are based on the following components:

as component A): nematic host component hydroquinone-bis- [4- (4-acryloylbutoxy) -benzoate](obtained according to Broer, d.j., Mol, g.n., Challa, g.; makromol. chem.1991, 192, 59).

As component B): one of the following chiral components:

a) diazim (di-2, 5- [ (4' -acryloyloxy) -benzoyl ] -isomannide, obtained according to EP 1149823, example 13)

b) AnABIs (2- [4- (acryloyloxy) -benzoyl ] -5- (4-methoxybenzoyl) -isosorbide, obtained according to EP1046692, example 3)

c) DiABIs (bis-2, 5- [4- (acryloyloxy) -benzoyl ] -isosorbide, obtained according to EP1046692, example 4)

Other preferred components B are cholesteryl methacrylate (obtained according to De Visser et al, J.Polym.Sci., A1 (9), 1893 (1971)).

The direction of circular polarization of the CLCP can be selected by appropriate selection of the above-mentioned optically active component B), in particular of a bivalent chiral residue Z, where Z is selected from dianhydrohexites (e.g. isosorbide or isomannide), hexoses, pentoses, binaphthyl derivatives, biphenyl derivatives, derivatives of tartaric acid and optically active diols, and where V is¹Or V²A C-C bond in the case of a cholesterol residue. Whereas the use of isosorbide derivatives, for example, leads to exclusively right-handed circularly polarized reflections, the use of cholesterol-containing derivatives or isomannide leads to exclusively left-handed circularly polarized reflections.

Preferred divalent residues of the invention are:

a) isosorbide:

b) isomannide:

the different achievable compositions are distinguished from one another essentially by the different contents of component B), wherein the color of the maximum reflection of the CLCP (i.e. the cholesteric pitch) can be set by the concentration of component B).

The optimum concentration of photoinitiator required for the polymerization varies with the content of component B); for the first irradiation step with a low UV dose, the useful concentration is between 0.00% and 5%, preferably between 0.25% and 2%, while for the second irradiation step with a high UV dose, the useful concentration is between 0.5% and 7%, preferably between 1% and 4%.

The concentration ranges of the photoinitiators in the various layers and the corresponding dosages of the curing agents (UV-radiation, electron beam, etc.) may vary to some extent from the values disclosed herein; however, the skilled person will retain the general principle of the present invention, namely to provide a sufficient amount of unreacted (reactive) groups in each layer, which can undergo the required interlayer crosslinking reaction in the subsequent curing step as well as in the final curing step. Curing by UV-radiation has proved to be the most practical option from an industrial point of view.

A method for producing a Cholesteric Liquid Crystal Polymer (CLCP) multilayer, wherein at least two layers of CLCPs that differ in at least one optical property are arranged on top of each other, comprising the steps of:

a) depositing a first coating L1 of a CLCP monomeric precursor material comprising crosslinkable groups onto a flexible carrier substrate;

b) orienting the CLCP coating;

c) partially curing the alignment layer of step a), thereby leaving a significant amount of crosslinkable groups in the layer;

d) optionally repeating steps a) to c) a selected number of times to provide additional layers L of CLCP monomeric precursor material comprising crosslinkable groups₂..L_n-1Deposited on the previous coating, oriented and partially cured;

e) applying a final coating L of a CLCP monomeric precursor material comprising crosslinkable groups_nDeposited on the previous coating;

f) orienting the CLCP coating;

g) curing the entire assembly thoroughly, thereby crosslinking substantially all of the crosslinkable groups throughout the coating;

the Cholesteric Liquid Crystal Polymer (CLCP) is characterized in that the at least two layers are chemically cross-linked together by a polymer network to form a mechanically unitary solid that can be comminuted into a pigment without deteriorating its internal structure, i.e. without delamination, and that has a cholesteric liquid crystal pitch dip at the interface between the at least two layers of the cholesteric liquid crystal polymer.

An alternative method for producing a Cholesteric Liquid Crystal Polymer (CLCP) multilayer, wherein at least two layers of CLCPs that differ in at least one optical property are arranged on top of each other, comprises the steps of:

b) freeze or evaporation drying the CLCP coating;

c) optionally repeating steps a) and b) a selected number of times to provide additional layers L of CLCP monomeric precursor material comprising crosslinkable groups₂..L_n-1Deposited on the previous coating;

d) applying a final coating L of a CLCP monomeric precursor material comprising crosslinkable groups_nDeposited on the previous coating;

e) freezing or drying the CLCP coating;

f) tempering the entire assembly to orient the deposited CLCP layer;

It is evident in this context that the coating can be applied from the molten state or from a solution. Curing can be carried out by UV-radiation, preferably by UV/A radiation. A lower UV radiation dose may be selected for the first layer and a higher UV radiation dose may be selected for the last layer. A lower amount of photoinitiator may be selected for the first layer and a higher amount of photoinitiator may be selected for the last layer. Curing may alternatively be by electron beam radiation.

In the context of the present invention, the curing of the polymer precursor is preferably carried out by UV-radiation, but other curing methods known to the skilled person, such as electron beam curing, ultrasonic curing, etc., may advantageously replace the UV curing method in certain applications. Common UV doses are 0.07-0.5J/cm²UV/A (as measured by a radiometer UV-Powerpuk from Eltosch, Hamburg, Germany).

According to a first embodiment and using a coating method known to the skilled person, such as knife coating or roll coating, a flexible carrier, for example a PET film or a continuous rubber, plastic or metal tape, is coated with a first layer of a cholesteric precursor mixture intended to produce a predetermined first optical property, preferably a reflection colour (spectral reflection maximum). The cholesterol precursor mixture contains a small amount of photoinitiator (in the range of 0-0.5%, preferably 0-0.25%). Using low dose (0.03-0.3J/cm)²Preferably 0.05 to 0.15J/cm²) And produces a polymeric cholesteric film which still contains reactive (active) groups but has stable color properties (frozen pitch). The average thickness of the first coating layer is 0.5 to 20 microns, preferably 1 to 10 microns.

If desired, additional intermediate layers of the same type as the first layer having independently selected optical properties can be applied to the coating thus obtained and hardened; for each intermediate layer, the amount of photoinitiator and the curing UV-radiation dose were kept low, as shown for the first layer. The average thickness of these coatings is 0.5 to 20 microns, preferably 1 to 10 microns.

In a final step, a final layer of the cholesteric monomer precursor mixture is applied on the already deposited coating layer, wherein said final layer is established with the aim of producing a predetermined optical property, preferably a reflection colour, the spectral reflection maximum of which differs in wavelength from the spectral reflection maximum of the first coating layer, preferably by at least 10-80nm, preferably 30-50 nm. The final coating contains a high concentration of photoinitiator (in the range of 0.2-3%, preferably 1.75%) and uses a relatively high UV-radiation dose (0.1-0.5J/cm)²) The polymerization is carried out. The average thickness of the final coating is 0.5 to 20 microns, preferably 1 to 10 microns.

The resulting CLCP film is completely resistant to delamination and behaves like a single layer in mechanical properties; i.e. no separation of the first and second layers is observed during the subsequent lift-off and comminution process to produce the pigment. This was confirmed by scanning electron micrographs that did not show any signs of phase boundaries throughout the thickness of the composite film. The transition from the first layer to the second layer can only be inferred from the change in the cholesteric structure, the slightly visible pitch.

In a second embodiment, a flexible support, such as a PET foil (or other suitable support), is coated sequentially with different liquid crystal melts in such a way that a first layer of the first melt is applied to the support through a first coating station a, which may be a doctor blade, sprayer or roll coater. The coating is thermally quenched (i.e. rapidly cooled below the solidification or glass transition temperature of the liquid crystal phase) and a second coating layer is applied to the first coating layer in the same process step, i.e. without cross-linking the previously applied layer, by means of a second coating station B (which may be a doctor blade, spray or roll coater), wherein said second coating layer is established with the aim of exhibiting optical properties, preferably reflection maxima, which preferably differ in wavelength from the reflection maxima of the first coating layer by at least 20 nm. The second coating is thermally quenched as indicated above and other coatings may be applied in the same process step by other coating stations C, D or the like, as desired.

The multilayer coating thus obtained is finally covered with a second PET foil (or other suitable covering foil) during the same process and enters a tempering zone, in which the temperature T is selected between 30 ℃ and 140 ℃, more preferably between 90 ℃ and 120 ℃, depending on the material used, the multilayer coating is returned to the liquid crystalline state and all the coatings previously applied adopt their particular pre-programmed pitch. The entire coating is then cross-linked (polymerized) thoroughly in one go by applying a suitable amount of UV-radiation (or electron beam radiation or other curing methods known to the skilled person).

The PET cover foil, similar to the PET substrate foil, serves to suppress the effect of atmospheric oxygen during oxygen-sensitive UV-polymerization reactions. The cover foil is applied on the CLCP coating immediately after the application of the last CLCP layer and before the UV polymerization stage.

The purpose of using the cover foil is two: on the one hand, the cover foil assists in excluding polymerization-inhibiting oxygen and, on the other hand, it serves to homogenize and orient the coating.

As known to the skilled person, the polymerized CLCP film is detached from the carrier and the cover foil by peeling, scraping, brushing or other operations. The resulting crude CLCP pieces are processed into pigments using known comminution operations, such as grinding with a hammer mill, impact mill, ball mill or jet mill, and classified by known separation methods, such as screen and sieving methods, in order to obtain pigments having the specified particle size, which have the d50 value in the range from 5 to 5000 microns specified for the application.

In a variant of this embodiment, a solution of CLCP monomeric precursor material intended to produce different optical properties (such as reflection wavelength) is coated onto a flexible PET carrier foil (or other suitable carrier) using coating methods known to the skilled person (such as roll coating, knife coating, curtain coating, etc.), and the solvent is evaporated after each coating step. The resulting "sandwich" is covered with a second PET foil (or other suitable cover foil) and allowed to return to the liquid crystalline state in the tempering zone where all of the previously applied coatings adopt their particular pre-programmed pitch. The entire coating is then thoroughly crosslinked (polymerized) all at once by applying a suitable amount of UV-radiation (or electron beam radiation and other curing methods known to the skilled person).

In yet another embodiment, a continuous strip of heat resistant material (e.g., steel, aluminum, etc.) is multi-layer coated with a melt or solution of CLCP precursor, wherein the melt or solution is established to produce optically different properties such as reflection wavelength, polarization, etc. Each coating was processed as described above.

The use of heat-resistant carrier tapes allows the processing of liquid-crystalline polymer precursors having a liquid-crystalline range at temperatures up to 400 ℃. Again, the crosslinking reaction is carried out according to methods known to the skilled worker, such as UV radiation or electron beam curing. At higher temperatures, inert conditions (excluding oxygen) must be chosen to prevent oxidative degradation of the active functional groups or products. An inert gas such as nitrogen, carbon dioxide or argon is used to reduce the oxygen concentration to the range of 5ppm to 1%, preferably to the range of 10 to 100 ppm.

When inert conditions are used in the curing step, the cover foil (second PET foil) for excluding oxygen is no longer required, even in the case of oxygen sensitive materials.

In the case of a carrier tape, the detachment of the CLCP layer from the substrate can also be carried out using high-pressure air jets, solid CO₂Jet flow, brushing process, etc.

Using the method of the invention, the CLCP multilayer of the invention is most preferably processed to pigments. For this purpose, the multilayer is peeled off from the carrier by means of suitable equipment, such as a peeling device or a peeling knife, to produce a crude CLCP sheet. These flakes are further comminuted to CLCP pigment using suitable tools such as grinding or cutting tools. The CLCP pigment is finally classified by a sifting and sieving operation.

Pigment flakes made according to the present invention have a thickness in the range of 0.1 to 50 microns and a diameter in the range of 10 to 1000 microns. Narrower sub-ranges are selected within these ranges according to the specific needs of each application. Most preferred are pigments having a platelet thickness of 0.5 to 6 microns and a platelet diameter of 1 to 200 microns.

The pigment particles obtained according to the present invention behave as a single solid in mechanical properties, but optically exhibit the combined characteristics of the individual layers that make up the pigment particles. It is therefore also possible, using the process of the invention, to produce CLCP pigments having reflective and/or other optical properties, which cannot be produced according to the prior art.

Obviously, an unconventional color shift can be produced, e.g. a color change from green to red-violet, whereas a conventional CLCP can at best exhibit a color shift from green to blue.

CLCP multilayers can similarly be produced, in which individual layers with different reflection wavelengths reflect light rays with different circular polarization directions. The resulting film, and the pigments produced therefrom, exhibit a first color to the unaided eye and different second and third colors when viewed through left-or right-handed circular polarizing filters, respectively.

The product produced according to the invention can be identified under scanning electron microscopy for a cholesteric liquid crystal pitch dip across the optical layer interface (see, e.g., the embodiment according to fig. 3 discussed below); the pitch is apparently responsible for the optical interference properties (reflected wavelength) of the cholesteric material. Referring to fig. 3, there is a first pitch having a pitch height of about 200 nm at a left portion of the layer, and a second pitch having a pitch height of about 130 nm at a right portion of the layer.

The CLCP pigments thus obtained are used in printing inks, as well as in lacquers and for the mass-colouring of plastic materials. In particular, the pigments of the invention can be formulated as printing inks for printing optical security markings, product security labels, etc., for example on banknotes, value documents, identity documents, tax banderoles, lottery tickets and transportation tickets. The optical security marking has the advantage that, in addition to the visible color shift effect upon changing the viewing angle, it also exhibits an invisible circular polarization effect, which can be made visible by means of a corresponding instrument.

In a particular embodiment of the security element, a first layer of the CLCP multilayer reflects a first color of left-handed circularly polarized light, e.g. green, and a second layer of the CLCP multilayer reflects a second color of right-handed circularly polarized light, e.g. red. There is a first visible color to the naked eye, consisting of two reflections, such as green and red, displayed by the security element; the resulting appearance was yellow. However, viewing under a left-hand circular polarizing filter, the same security element will display green, while viewing under a right-hand circular polarizing filter, it will correspondingly display red.

The pigments of the invention are preferably used in printing inks for screen printing, flexographic printing and gravure printing processes, however, offset, copper gravure and tampogaphic printing processes are also conceivable.

In addition to their use in printing inks, the pigments according to the invention can also be used in industrial and automotive coating lacquers, and also in the mass colouring of cosmetics and plastics and masterbatches for the plastics industry.

The multilayer Cholesteric Liquid Crystal Polymers (CLCP) of the invention can be used in the field of security documents, in the graphic industry, in coating compositions or in cosmetics.

The flake pigments according to the invention can be used in the field of security documents, in the graphic industry, in coating compositions, in-mold applications or in cosmetics.

The invention also claims any object containing the flake-like pigments disclosed herein. It is noted that flake pigments can be used in printing ink and coating compositions which can be used in particular for the protection of security documents, such as banknotes, value documents, identity documents, tax banderoles, access cards, transport tickets or product security labels.

Cholesteric Liquid Crystal Polymer (CLCP) multilayers of the invention and pigments produced therefrom can also be used in a variety of technical fields according to the following non-exclusive list: automotive paint, OEM and refurbishment; dip coating (e.g., for candles); coloring the plastic by compounding or compounding; in-mold applications (printing on PC film placed on the surface of a three-dimensional plastic part); cosmetic applications such as nail polish, eye shadow, lotion, mascara, foundation, cream, compact, gel, hair spray, and the like; powder coating; water-based or solvent-based industrial coatings; coatings for plastics and metals; gel coats (e.g., for boats and yachts); printing inks (screen printing inks, flexographic printing, gravure (gravure), gravure (intaglio), etc.); packaging; security applications such as security threads, security signs, product security labels, seals, hot embossing features, etc.; security features on banknotes, vouchers, ID documents, certificates, (transport) tickets; paints and coatings for consumer electronics; paints and coatings for sports equipment; furniture paints and coatings; glass paint; building paint; a fishing bait; product identity characteristics, aerosol paint (self-made); a traffic sign; advertising; machine-readable security features (color + polarization); an entertainment device; vinyl artificial leather (seat); applique; an aviation coating.

The invention will now be further illustrated by means of non-limiting exemplary embodiments and the accompanying drawings:

FIG. 1 shows a scanning electron micrograph of a two-layer pigment according to the present invention; including annotations for the general physical dimensions of the pigment particles.

FIG. 2 shows scanning electron micrographs of some common fracture regions of the two-layer pigments of the present invention, with the noted thickness values. No delamination was seen at the layer boundaries.

Fig. 3 shows a scanning electron micrograph of the edges of a two-layer pigment particle according to the invention, which illustrates the fact that a) no phase boundary (which would appear as a fracture irregularity) is seen between the two layers, and b) there are two layers with different optical properties. The helical pitch of the cholesteric structure is visible as a fine pitch through the sheet. There was a sharp visible span density dip in the center of the patch (corresponding to a helical pitch change; about 200 nm in the left part of the image and about 130 nm in the right part of the image).

FIG. 4 shows a scanning electron microscope photomicrograph of a multilayer pigment flake prepared according to the prior art method (Dobrusskin et al, WO 95/08786); this material clearly shows a definite mechanical phase boundary between the different sublayers and tends to decompose into its individual layers at the fracture zone.

FIG. 5 shows a scanning electron microscope photograph of a closer field of view of the prior art pigment flake fracture region of FIG. 4; regular breaks at the boundaries of the individual sub-layers were observed, indicating that the pigment flakes are susceptible to decomposition into their individual layers under mechanical stress (pigment preparation, incorporation of ink, printing).

Fig. 6 shows the reflection spectrum of a CLCP bilayer of the invention similar to example 11 in table 1: (a) a first layer after application and partial UV-curing; a reflection maximum at a wavelength of about 700 nm; (b) a second layer after application and partial UV-curing; a reflection maximum at a wavelength of about 560 nm; (c) a second layer on the first layer after thorough UV-curing; the reflection maxima are at wavelengths of about 550nm and 725 nm.

Fig. 7 shows a scanning electron micrograph of the edges of a three-layer pigment particle produced according to the prior art method (US2005/0266158a 1) illustrating the progressive pitch change across the particle.

Fig. 8 shows the transmission spectrum of a three-layer pigment particle produced according to the prior art method (US2005/0266158a 1), which illustrates the presence of three different optical layers (the corresponding reflection spectrum can be deduced by inverting the curve).

Fig. 9 shows the evolution of the pitch height across the edges of the pigment particles of the following pigments: a) pigments produced according to the prior art method (US2005/0266158a 1) and b) pigments produced according to the invention.

Examples

Starting materials used in examples 1 to 15

In the synthesis of the pigments of examples 1-15, the following starting materials were used. In table 1 at the end of the example section, which component is used in which example is indicated by a bold number.

i) Nematic main component (component A in the above formula): hydroquinone-bis- [4- (4-acryloylbutoxy) -benzoate ], (1), (obtained according to Broer, d.j., Mol, g.n., Challa, g.; makromol. chem.1991, 192, 59);

ii) a chiral component (component B in the above formula):

AnABIs, 2- [4- (acryloyloxy) -benzoyl ] -5- (4-methoxybenzoyl) -isosorbide, (2), (obtained according to EP1046692, example 3);

dianbis, bis-2, 5- [4- (acryloyloxy) -benzoyl ] -isosorbide, (3), (obtained according to EP1046692, example 4);

DiaBli, bis-2, 5- [ (4' -acryloyloxy) -benzoyl ] -isomannide, (4), (obtained according to EP 1149823, example 13) or cholesteryl methacrylate (5), (obtained according to De Visser et al, J.Polym.Sci., A1 (9), 1893 (1971));

iii) polymerization stabilizer 2, 6-di-tert-butyl-4- (dimethylamino-methyl) -phenol (6) ((ii) a salt of N, N-dimethylformamide703，Ethyl Corp.，Baton Rouge，LA 70801)；

iv) photoinitiator (7)819(Ciba Specialty Chemicals GmbH，Lampertsheim)。

General Synthesis of the pigments of examples 1-15

According to the weight ratios given in the examples (relative to 100 parts of main component), nematic main component 1 and the corresponding chiral compound 2,3, 4 or 5 and about 300ppm of stabilizer 6 are mixed together in a heatable container and melted until a clear liquid is obtained. The melt is homogenized with a stirrer and finally the photoinitiator 7 is added with stirring. The separate stirring addition of the photoinitiator 7 as the last component according to the weight ratios given in the examples serves to prevent premature thermally induced crosslinking of the mixture. The composition thus obtained is used as a material for a cholesteric layer to be produced on a substrate.

The amounts of the compounds used in the examples are given in table 1.

The LC-mixtures prepared as described above were applied to a pre-tempered flexible polyethylene terephthalate (PET) carrier substrate in the layer thicknesses shown in table 1 below by means of a roll coater according to the outlined method. The coating and curing conditions for each example are also shown in table 1 below.

Generally, in the two-stage coating process described above, on a substrate, a first cholesteric layer is applied directly onto the PET substrate, and subsequently, a second cholesteric layer is applied onto the first layer. After a defined diffusion time (i.e. the residence time of the double layer in the tempering equipment), the entire coating is UV-polymerized.

The layer thickness of all applied layers is controlled individually on the basis of the amount of LC-mixture used per coated area. After the coating is complete, the layer thickness is measured by means of a layer thickness measuring instrument Supermess (Mahr GmbH, D-37073)) And (6) performing cross-over inspection. The wavelength of maximum reflection is obtained from the transmission spectrum of the individual layers by means of a UV/VIS spectrometer (Perkin Elme, model Lambda 19, Ueberlingen, Germany). The values obtained are summarized in table 1 below.

To suppress the influence of atmospheric oxygen during oxygen-sensitive UV-polymerization, a PET cover foil similar to the PET base foil was used. The cover foil is applied on the CLCP coating immediately after the application of the last CLCP layer and before the UV polymerization stage.

After the application of the layers, the CLCP-coated and PET foil-covered substrate was passed through a tempering/orientation tunnel, where the substrate was exposedAt a temperature of 90 ℃ to 125 ℃, typically about 110 ℃. Due to the constant length of the tunnel, the time for the alignment of the liquid crystal coating is determined by the speed of passage. At the end of the tunnel, the oriented liquid crystal layer was passed through a mercury UV lamp (dose 0.07-0.5J/cm)²UV/A range).

By using a reduced dose of UV radiation and a lower concentration of photoinitiator, the first layer is not completely crosslinked. The cover foil is removed and the cured first coating on the PET foil substrate is coated with a second layer of an LC-mixture having a reflection wavelength which differs from the reflection wavelength of the first layer by at least 20 nm.

After the second coating operation and the corresponding application of the cover foil, use is made of a foil material in the range from 0.07 to 0.5J/cm²UV dose in the UV/A range, the entire coating (i.e. the resulting multilayer) is subjected to a second UV polymerization.

Thereafter, the resulting "sandwich" of substrate, CLCP bilayer and cover foil is separated and the CLCP bilayer is peeled off from the PET foil (substrate and/or cover foil) by means of a knife. The exfoliated CLCP material, which is present in the form of coarse flakes, is processed to pigments by grinding on an air jet mill (Hokosawa-Alpine company, Augsburg, germany) followed by a screen/sieving to give CLCP-pigments having a particle size d50 of 18 to 35 μm. Particle size was determined using a particle size analyzer HELOS (measurement of dispersion in water) from Sympatec GmbH, Clausthal-Zellerfeld. Fig. 1, 2 and 3 show electron micrographs of the pigment thus obtained.

Scanning electron micrographs of the cracked edges of the inventive bilayer CLCP film (fig. 2) illustrate the fact that a) there is no mechanical phase boundary between the two layers (which would appear as a fracture kink), and b) there are two layers with different optical properties. The helical pitch of the cholesteric structure is visible in the electron micrograph as a fine pitch through the thickness of the film. There was a sharp visible step in the span density (corresponding to a helical pitch change) in the center of the film (fig. 3: about 200 nm in the left part of the image and about 130 nm in the right part of the image). The inventive material is characterized in that the helical pitch changes abruptly at the interface of the optical layers of different properties; the pitch changes from a first value to a second value within a single pitch height so that no intermediate pitch region is observed.

The span visible under an electron microscope corresponding to a cholesteric structure is not a mechanical layer structure in terms of the presence of a layer along which the flakes may break; indeed, such fractures have not been observed in the materials of the present invention. The observed span is due to differential charging effects of the ordered cholesteric material, which can be generated using certain experimental conditions in taking SEM pictures.

For comparison, multilayer pigments prepared according to the prior art method (WO 95/08786) clearly show defined mechanical phase boundaries between the different sublayers and tend to decompose into their individual layers at these pre-set fracture zones, as shown in fig. 4 and 5.

Fig. 6 shows the reflection spectra of two individual CLCP layers (a, b) differing in reflection maximum, and the reflection spectrum of the corresponding CLCP bilayer (c) of the invention showing the two reflection maxima of (a) and (b).

To represent the differences between the products produced according to the method of the invention and the products produced according to the prior art method (US2005/0266158a1, Pokorny et al) for comparison, cholesteric multilayers were produced according to Pokorny et al by applying a single thick liquid layer comprising a CLC polymer, a CLC-monomer, and a solvent. The layer thus applied is subsequently subjected to i) a first partial evaporation drying, ii) a first partial UV-curing, iii) a second thorough evaporation drying and iv) a second thorough UV-curing.

Fig. 7 shows a scanning electron micrograph of a cross-section of the resulting 8 micron thick CLCP layer. There is no abrupt pitch change of the cholesteric liquid crystal, but there is a gradual increase in pitch from bottom to top, followed by a steeper pitch but also a gradual decrease. The cholesteric pitch evolves smoothly through the layer without significant steps.

FIG. 8 shows the resulting transmission spectra, which are similar to those reported by Pokorny et Al (FIGS. 16, 17 of US2005/0266158 Al) and indicate the presence of three different optical layers.

To illustrate the differences observed, individual pitch heights across the multiple layers were measured in a prior art SEM image (fig. 7) and an inventive SEM image (fig. 3).

Figure 9a shows a gradual increase and decrease in pitch height across multiple layers produced according to Pokorny et al.

Figure 9b shows a sudden decrease in pitch height across the multilayer produced according to the present invention. The change from the first pitch to the second pitch occurs substantially within a single pitch height such that no intermediate pitch height region is observed.

From a thermodynamic point of view it is clear that the partial evaporation method as used in the method of Pokorny et al has to produce a gradual change in pitch height, since the conditions across the cholesteric layer are not uniform if evaporation at the surface is involved. In the method of the invention, evaporation of the volatile components is not involved, and layers with predetermined properties are applied on top of each other, which results in a sudden change of properties at the layer boundary.

Table 1:

the numbering of the chiral compounds refers to the numbering shown herein.

For a given embodiment, the required UV/A radiation dose is about 0.3J/cm²Corresponding to the indicated 100% value of the uv power. Lower percentage values in the table refer to correspondingly lower UV/a doses.

Preparation of paints containing the pigments according to the invention

The CLCP pigment obtained as described above is stirred into a Clear coating composition (e.g., Tinted Clear Additive Deltron 941, PPG Industries, UK-. Suffolk, IP142AD) at a 3% weight ratio.

Effect coatings on paper supports Using the pigments according to the invention

The coating compositions of the preceding examples were applied to a black glossy paper support by means of a film applicator (Erichsen, D-58675Hemer) using a gap height of 180 μm and a coating speed of 10mm per second. After drying at room temperature for a period of 10 minutes, the coated substrate was dried at 80 ℃ for 1 hour. The reflection spectrum of the dried lacquer was determined with a colorimeter CM508/D from Minolta (D-22923Ahrensburg) and the wavelength of the corresponding maximum reflection is mentioned in the table.

Polarization effects of the embodiment of example 15

The effect coating obtained as described above using the pigment of example 15 was visually observed under left-and right-hand circular polarizing filters (e.g. obtainable from Schneider-Kreuznach, Bad Kreuznach, germany). Under a left-hand circular polarizing filter, a red color is observed at a vertical viewing angle, and under a right-hand circular polarizing filter, a blue color is observed at a vertical viewing angle. Without the circular polarizing filter, a blue-violet color is observed at vertical viewing angles, which gradually changes to red as the viewing angle slope increases.

Claims

1. A method for preparing a Cholesteric Liquid Crystal Polymer (CLCP) multilayer, wherein at least two layers of CLCPs that differ in at least one optical property are arranged on top of each other, the method comprising the steps of:

a) first coating L of a CLCP monomeric precursor material comprising crosslinkable groups₁Depositing onto a flexible carrier substrate;

b) orienting the CLCP coating;

c) partially curing the oriented layer of step a) leaving an amount of crosslinkable groups in the layer to chemically inter-layer crosslink with an adjacent coating layer through the polymer network;

f) orienting the CLCP coating;

g) the entire assembly is cured thoroughly, thereby crosslinking substantially all of the crosslinkable groups throughout the coating and forming a mechanically unique solid body that can be comminuted into a pigment without degrading its internal structure.

2. A method for preparing a Cholesteric Liquid Crystal Polymer (CLCP) multilayer, wherein at least two layers of CLCPs that differ in at least one optical property are arranged on top of each other, the method comprising the steps of:

b) freeze or evaporation drying the CLCP coating;

e) freezing or drying the CLCP coating;

f) tempering the entire assembly to orient the deposited CLCP layer;

3. The method of claim 1 or 2, wherein the coating is applied from a molten state.

4. The method of claim 1 or 2, wherein the coating is applied from solution.

5. The method of claim 1 or 2, wherein the curing is performed by UV-radiation.

6. The method of claim 5, wherein a lower UV radiation dose is selected for the first layer and a higher UV radiation dose is selected for the last layer.

7. The method of claim 1 or 2, wherein the amount of photoinitiator contained in the CPLC precursor material is selected to be lower in the first layer and higher in the last layer.

8. The method of claim 1 or 2, wherein the curing is performed by electron beam radiation.

9. A multilayer of Cholesteric Liquid Crystal Polymer (CLCP) in which at least two layers of CLCP differing in at least one optical property are arranged on top of each other, characterized in that the at least two layers are chemically inter-layer crosslinked by means of a polymer network, thereby forming a mechanically unique solid body which can be comminuted into a pigment without deteriorating its internal structure and which has a cholesteric liquid crystal pitch dip at the interface between the at least two layers of the cholesteric liquid crystal polymer.

10. The Cholesteric Liquid Crystal Polymer (CLCP) multilayer of claim 9, wherein the CLCP comprises components a) and B), wherein

A) Is 20 to 99.5 wt.% of at least one or several three-dimensional crosslinkable compounds of the average general formula (1)

Y¹-A¹-M¹-A²-Y² (1)

Wherein

Y¹、Y²Are identical or different and represent a polymerizable group selected from acrylate, methacrylate, epoxy, isocyanate, hydroxyl, vinyl ether or vinyl residues;

M¹having the formula-R¹-X¹-R²-X²-R³-X³-R⁴-；

Wherein

wherein

B¹To B³Are identical or different substituents selected from the group consisting of hydrogen, C₁-C₂₀Alkyl radical, C₁-C₂₀-alkoxy, C₁-C₂₀Alkylthio radical, C₁-C₂₀-alkylcarbonyl group, C₁-C₂₀Alkoxycarbonyl, C₁-C₂₀-alkylthio carbonyl, -OH, -F, -Cl, -Br, -I, -CN, -NO₂Formyl, acetyl and alkyl-, alkoxy-or alkylthio-residues having a chain interrupted by ether oxygen, thioether, sulfur or ester groups, having 1 to 20 carbon atoms;

B) is 0.5 to 80 wt.% of at least one chiral compound of the average formula (2)

V¹-A¹-W¹-Z-W²-A²-V²(2)

Wherein

V¹，V²Identical or different and represent the following residues: acrylate, methacrylate, epoxy, vinyl ether, vinyl, isocyanate, C₁-C₂₀Alkyl radical, C₁-C₂₀-alkoxy, C₁-C₂₀Alkylthio radical, C₁-C₂₀-alkylcarbonyl group, C₁-C₂₀Alkoxycarbonyl, C₁-C₂₀-alkylthio carbonyl, -OH, -F, -Cl, -Br, -I, -CN, -NO₂Formyl, acetyl and alkyl-, alkoxy-or alkylthio-residues having a chain of 1 to 20 carbon atoms interrupted by ether oxygen, thioether sulfur or an ester group, or a cholesterol residue;

A¹、A²as described above;

W¹、W²having the formula-R¹-X¹-R²-X²-R³-；

Wherein

R¹To R³As described above, and wherein R²Or R²-X²Or X¹-R²-X²-R³May also be a C-C bond;

X¹、X²as described above;

z is a divalent chiral residue selected from dianhydroxites, hexoses, pentoses, binaphthyl derivatives, biphenyl derivatives, tartaric acid derivatives and optically active diols, and wherein V is¹Or V²A C-C bond in the case of a cholesterol residue.

11. The multilayers of claim 10, wherein component B) is selected from the group consisting of AnABIs- (2- [4- (acryloyloxy) -benzoyl ] -5- (4-methoxybenzoyl) -isosorbide), DiABIs (di-2, 5- [4- (acryloyloxy) -benzoyl ] -isosorbide) or DiABIm (di-2, 5- [ (4' -acryloyloxy) -benzoyl ] -isomannide).

12. The multilayer of any of claims 9-11, wherein the different optical properties are selected from the group consisting of wavelength of maximum reflection, circular polarization state of reflected light, optical absorption characteristics.

13. The multilayer of any of claims 9-11, wherein the multilayer has narrow-band spectral features that are imperceptible to the unaided human eye.

14. The multilayer of any one of claims 9-11, further comprising an additive having non-optical properties selected from the group consisting of magnetic particles, radio frequency resonance particles, and forensic markers.

15. A flake pigment for printing or coating applications, obtained by comminuting a multilayer of a Cholesteric Liquid Crystal Polymer (CLCP) according to any one of claims 9 to 14.

16. A platelet-shaped pigment according to claim 15 wherein the median pigment size d50 is from 5 to 5000 microns.

17. Use of a multilayer of Cholesteric Liquid Crystal Polymers (CLCP) according to any of claims 9 to 14 for the preparation of a platelet-shaped pigment according to claim 15 or 16.

18. Use of a flake pigment according to any of claims 15 or 16 in the field of security documents, in the graphic industry, in coating compositions, in-mold applications or in cosmetics.

19. An object or a printing ink or coating composition comprising a flake pigment according to any of claims 15 or 16.

20. Use of the printing ink or coating composition of claim 19 for protecting a security document selected from banknotes, value documents, identity documents, tax banderoles, access cards, transport documents or product security labels.